The present invention relates to spherical beads comprising at least one metal and/or semimetal oxide, having a diameter in the range from 10 to 120 μm, a BET surface area in the range from 400 to 800 m2/g and a pore volume in the range from 0.3 to 3.0 cm3/g, wherein the diameter at no point of a given bead deviates by more than 10% from the average diameter of said bead and the surface of said bead is substantially smooth, to a process for producing these spherical beads and to the use of spherical beads as catalysts or catalyst carriers.
The present invention relates more particularly to spherical beads comprising SiO2 (silica).
Silica particles and methods of making them are already known from the prior art. U.S. Pat. No. 2,757,073 discloses a process for making pulverulent silica gel wherein a dispersion of a silica gel sol is initially charged in a water-saturated organic solvent, for example butanol, and droplets of the silica gel sol are formed by vigorous stirring and converted by addition of ammonia into silica gel. The gel thus obtained is broken into small particles after hardening and the small particles are washed with water and with acetone to remove substantially all the water. U.S. Pat. No. 2,757,073 further discloses that such pulverulent silica can be used as a filler in materials such as rubber or as an intermediate for the preparation of coated silica products. The diameter of the silica beads thus obtained is less than 50 nm.
U.S. Pat. No. 2,921,839 discloses a method of producing finely divided silica particles by precipitation. To this end, an aqueous solution of an alkali metal silicate is admixed with an organic solvent and subsequently with an acid. After the organic phase has been separated off, the silica gel beads obtained are dried by azeotropic distillation. The method described in U.S. Pat. No. 2,921,839 provides silica gel particles having a diameter in the range from 10 to 1000 μm. U.S. Pat. No. 2,921,839 does not disclose any silica gel particles that are notable for a particularly high smoothness on their surface.
U.S. Pat. No. 3,489,516 discloses a method of producing silica beads by polymerization of xNa2O·ySiO2 in a dispersed organic phase in an aqueous medium by addition of an acid. The silica beads thus produced have a BET surface area in the range from 700 to 1100 m2/g or in the range from 300 to 600 m2/g. These silica beads can be used as catalysts.
There is no prior art describing silica particles, or their methods of making, that display a combination of diameter, BET surface area, pore volume, polydispersity and surface smoothness that is particularly suitable according to the present invention and that appears to make these particles particularly suitable for use as catalysts in polymerization reactions, for example for producing polypropylene or polyethylene. Nor is there prior art describing a process that provides spherical beads that display the recited advantageous combination of various parameters.